It is not a common extention. I have yet to come across more than a handful of sites using it. Also, the native population of the Indian Ocean Territories that use .io, has been evacuated and lives shattered across many other countries now (including a large number in the UK and Mauritius). I doubt these exiles would use .io over the extention of their country of residence, and with no plans to allow population again soon on the islands themselves I doubt this extention has much value.